<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        body {
            height: 2000px;
        }
        
        .back {
            position: fixed;
            width: 50px;
            height: 50px;
            background-color: pink;
            right: 30px;
            bottom: 100px;
            display: none;
        }
        
        .container {
            width: 900px;
            height: 500px;
            background-color: skyblue;
            margin: 400px auto;
        }
    </style>
  
</head>
<script src="./jquery.min.js"></script>
<body>
    <div class="back">返回顶部</div>
    <div class="container">
    </div>
    <script>
       $(function() {
           var boxTop = $(".container").offset().top;
           $(window).scroll(function() {
               if($(document).scrollTop() >= boxTop){
                   $(".back").fadeIn();
               }else{
                $(".back").fadeOut();
               }
           })

        //  返回顶部
        $(".back").click(function() {
            $("body,html").stop().animate({
                scrollTop:0,
            })
        })
       })
     
    </script>
</body>

</html>